High performance interconnect link layer

ABSTRACT

Transaction data is identified and a flit is generated to include three or more slots and a floating field to be used as an extension of any one of two or more of the slots. In another aspect, the flit is to include two or more slots, a payload, and a cyclic redundancy check (CRC) field to be encoded with a 16-bit CRC value generated based on the payload. The flit is sent over a serial data link to a device for processing, based at least in part on the three or more slots.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a continuation (and claims the benefit of priorityunder 35 U.S.C. § 120) of U.S. application Ser. No. 15/264,680, filedSep. 13, 2016 and entitled HIGH PERFORMANCE INTERCONNECT LINK LAYER,which is a continuation of U.S. application Ser. No. 13/976,947, with aneffective filing date of Mar. 28, 2013 and entitled HIGH PERFORMANCEINTERCONNECT LINK LAYER, now U.S. Pat. No. 9,479,196, which applicationis a national stage application under 35 U.S.C. § 371 of PCTInternational Application Serial No. PCT/US2013/034341, filed on Mar.28, 2013 and entitled HIGH PERFORMANCE INTERCONNECT LINK LAYER, whichapplication claims the benefit of priority to U.S. Provisional PatentApplication Ser. No. 61/717,091 filed on Oct. 22, 2012 and entitledMETHOD, APPARATUS, SYSTEM FOR A HIGH PERFORMANCE INTERCONNECTARCHITECTURE. The disclosures of the prior applications are consideredpart of and are hereby incorporated by reference in their entirety inthe disclosure of this application.

FIELD

The present disclosure relates in general to the field of computerdevelopment, and more specifically, to software development involvingcoordination of mutually-dependent constrained systems.

BACKGROUND

Advances in semi-conductor processing and logic design have permitted anincrease in the amount of logic that may be present on integratedcircuit devices. As a corollary, computer system configurations haveevolved from a single or multiple integrated circuits in a system tomultiple cores, multiple hardware threads, and multiple logicalprocessors present on individual integrated circuits, as well as otherinterfaces integrated within such processors. A processor or integratedcircuit typically comprises a single physical processor die, where theprocessor die may include any number of cores, hardware threads, logicalprocessors, interfaces, memory, controller hubs, etc.

As a result of the greater ability to fit more processing power insmaller packages, smaller computing devices have increased inpopularity. Smartphones, tablets, ultrathin notebooks, and other userequipment have grown exponentially. However, these smaller devices arereliant on servers both for data storage and complex processing thatexceeds the form factor. Consequently, the demand in thehigh-performance computing market (i.e. server space) has alsoincreased. For instance, in modern servers, there is typically not onlya single processor with multiple cores, but also multiple physicalprocessors (also referred to as multiple sockets) to increase thecomputing power. But as the processing power grows along with the numberof devices in a computing system, the communication between sockets andother devices becomes more critical.

In fact, interconnects have grown from more traditional multi-drop busesthat primarily handled electrical communications to full blowninterconnect architectures that facilitate fast communication.Unfortunately, as the demand for future processors to consume at evenhigher-rates corresponding demand is placed on the capabilities ofexisting interconnect architectures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified block diagram of a system including aserial point-to-point interconnect to connect I/O devices in a computersystem in accordance with one embodiment:

FIG. 2 illustrates a simplified block diagram of a layered protocolstack in accordance with one embodiment:

FIG. 3 illustrates an embodiment of a serial point-to-point link.

FIG. 4 illustrates embodiments of potential High PerformanceInterconnect (HPI) system configurations.

FIG. 5 illustrates an embodiment of a layered protocol stack associatedwith HPI.

FIG. 6 illustrates a representation of an example multi-slot flit.

FIG. 7 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 8 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 9 illustrates a representation of an example flit sent over anexample twenty-lane data link.

FIG. 10 illustrates a representation of use of an example floatingpayload field of an example multi-slot flit.

FIG. 11 illustrates an embodiment of a block for an example computingsystem.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth,such as examples of specific types of processors and systemconfigurations, specific hardware structures, specific architectural andmicro architectural details, specific register configurations, specificinstruction types, specific system components, specific processorpipeline stages, specific interconnect layers, specificpacket/transaction configurations, specific transaction names, specificprotocol exchanges, specific link widths, specific implementations, andoperation etc. in order to provide a thorough understanding of thepresent invention. It may be apparent, however, to one skilled in theart that these specific details need not necessarily be employed topractice the subject matter of the present disclosure. In otherinstances, well detailed description of known components or methods hasbeen avoided, such as specific and alternative processor architectures,specific logic circuits/code for described algorithms, specific firmwarecode, low-level interconnect operation, specific logic configurations,specific manufacturing techniques and materials, specific compilerimplementations, specific expression of algorithms in code, specificpower down and gating techniques/logic and other specific operationaldetails of computer system in order to avoid unnecessarily obscuring thepresent disclosure.

Although the following embodiments may be described with reference toenergy conservation, energy efficiency, processing efficiency, and so onin specific integrated circuits, such as in computing platforms ormicroprocessors, other embodiments are applicable to other types ofintegrated circuits and logic devices. Similar techniques and teachingsof embodiments described herein may be applied to other types ofcircuits or semiconductor devices that may also benefit from suchfeatures. For example, the disclosed embodiments are not limited toserver computer system, desktop computer systems, laptops, Ultrabooks™,but may be also used in other devices, such as handheld devices,smartphones, tablets, other thin notebooks, systems on a chip (SOC)devices, and embedded applications. Some examples of handheld devicesinclude cellular phones, Internet protocol devices, digital cameras,personal digital assistants (PDAs), and handheld PCs. Here, similartechniques for a high-performance interconnect may be applied toincrease performance (or even save power) in a low power interconnect.Embedded applications typically include a microcontroller, a digitalsignal processor (DSP), a system on a chip, network computers (NetPC),set-top boxes, network hubs, wide area network (WAN) switches, or anyother system that can perform the functions and operations taught below.Moreover, the apparatus', methods, and systems described herein are notlimited to physical computing devices, but may also relate to softwareoptimizations for energy conservation and efficiency. As may becomereadily apparent in the description below, the embodiments of methods,apparatus', and systems described herein (whether in reference tohardware, firmware, software, or a combination thereof) may beconsidered vital to a “green technology” future balanced withperformance considerations.

As computing systems are advancing, the components therein are becomingmore complex. The interconnect architecture to couple and communicatebetween the components has also increased in complexity to ensurebandwidth demand is met for optimal component operation. Furthermore,different market segments demand different aspects of interconnectarchitectures to suit the respective market. For example, serversrequire higher performance, while the mobile ecosystem is sometimes ableto sacrifice overall performance for power savings. Yet, it is asingular purpose of most fabrics to provide highest possible performancewith maximum power saving. Further, a variety of different interconnectscan potentially benefit from subject matter described herein. Forinstance, the Peripheral Component Interconnect (PCI) Express (PCIe)interconnect fabric architecture and QuickPath Interconnect (QPI) fabricarchitecture, among other examples, can potentially be improvedaccording to one or more principles described herein, among otherexamples.

FIG. 1 illustrates one embodiment of a fabric composed of point-to-pointLinks that interconnect a set of components is illustrated. System 100includes processor 105 and system memory 110 coupled to controller hub115. Processor 105 can include any processing element, such as amicroprocessor, a host processor, an embedded processor, a co-processor,or other processor. Processor 105 is coupled to controller hub 115through front-side bus (FSB) 106. In one embodiment, FSB 106 is a serialpoint-to-point interconnect as described below. In another embodiment,link 106 includes a serial, differential interconnect architecture thatis compliant with different interconnect standard.

System memory 110 includes any memory device, such as random accessmemory (RAM), non-volatile (NV) memory, or other memory accessible bydevices in system 100. System memory 110 is coupled to controller hub115 through memory interface 116. Examples of a memory interface includea double-data rate (DDR) memory interface, a dual-channel DDR memoryinterface, and a dynamic RAM (DRAM) memory interface.

In one embodiment, controller hub 115 can include a root hub, rootcomplex, or root controller, such as in a PCIe interconnectionhierarchy. Examples of controller hub 115 include a chipset, a memorycontroller hub (MCH), a northbridge, an interconnect controller hub(ICH) a southbridge, and a root controller/hub. Often the term chipsetrefers to two physically separate controller hubs, e.g., a memorycontroller hub (MCH) coupled to an interconnect controller hub (ICH).Note that current systems often include the MCH integrated withprocessor 105, while controller 115 is to communicate with I/O devices,in a similar manner as described below. In some embodiments,peer-to-peer routing is optionally supported through root complex 115.

Here, controller hub 115 is coupled to switch/bridge 120 through seriallink 119. Input/output modules 117 and 121, which may also be referredto as interfaces/ports 117 and 121, can include/implement a layeredprotocol stack to provide communication between controller hub 115 andswitch 120. In one embodiment, multiple devices are capable of beingcoupled to switch 120.

Switch/bridge 120 routes packets/messages from device 125 upstream, i.e.up a hierarchy towards a root complex, to controller hub 115 anddownstream, i.e. down a hierarchy away from a root controller, fromprocessor 105 or system memory 110 to device 125. Switch 120, in oneembodiment, is referred to as a logical assembly of multiple virtualPCI-to-PCI bridge devices. Device 125 includes any internal or externaldevice or component to be coupled to an electronic system, such as anI/O device, a Network Interface Controller (NIC), an add-in card, anaudio processor, a network processor, a hard-drive, a storage device, aCD/DVD ROM, a monitor, a printer, a mouse, a keyboard, a router, aportable storage device, a Firewire device, a Universal Serial Bus (USB)device, a scanner, and other input/output devices. Often in the PCIevernacular, such as device, is referred to as an endpoint. Although notspecifically shown, device 125 may include a bridge (e.g., a PCIe toPCI/PCI-X bridge) to support legacy or other versions of devices orinterconnect fabrics supported by such devices.

Graphics accelerator 130 can also be coupled to controller hub 115through serial link 132. In one embodiment, graphics accelerator 130 iscoupled to an MCH, which is coupled to an ICH. Switch 120, andaccordingly I/O device 125, is then coupled to the ICH. I/O modules 131and 118 are also to implement a layered protocol stack and associatedlogic to communicate between graphics accelerator 130 and controller hub115. Similar to the MCH discussion above, a graphics controller or thegraphics accelerator 130 itself may be integrated in processor 105.

Turning to FIG. 2 an embodiment of a layered protocol stack isillustrated. Layered protocol stack 200 can includes any form of alayered communication stack, such as a QPI stack, a PCIe stack, a nextgeneration high performance computing interconnect (HPI) stack, or otherlayered stack. In one embodiment, protocol stack 200 can includetransaction layer 205, link layer 210, and physical layer 220. Aninterface, such as interfaces 117, 118, 121, 122, 126, and 131 in FIG.1, may be represented as communication protocol stack 200.Representation as a communication protocol stack may also be referred toas a module or interface implementing/including a protocol stack.

Packets can be used to communicate information between components.Packets can be formed in the Transaction Layer 205 and Data Link Layer210 to carry the information from the transmitting component to thereceiving component. As the transmitted packets flow through the otherlayers, they are extended with additional information used to handlepackets at those layers. At the receiving side the reverse processoccurs and packets get transformed from their Physical Layer 220representation to the Data Link Layer 210 representation and finally(for Transaction Layer Packets) to the form that can be processed by theTransaction Layer 205 of the receiving device.

In one embodiment, transaction layer 205 can provide an interfacebetween a device's processing core and the interconnect architecture,such as Data Link Layer 210 and Physical Layer 220. In this regard, aprimary responsibility of the transaction layer 205 can include theassembly and disassembly of packets (i.e., transaction layer packets, orTLPs). The translation layer 205 can also manage credit-based flowcontrol for TLPs. In some implementations, split transactions can beutilized, i.e., transactions with request and response separated bytime, allowing a link to carry other traffic while the target devicegathers data for the response, among other examples.

Credit-based flow control can be used to realize virtual channels andnetworks utilizing the interconnect fabric. In one example, a device canadvertise an initial amount of credits for each of the receive buffersin Transaction Layer 205. An external device at the opposite end of thelink, such as controller hub 115 in FIG. 1, can count the number ofcredits consumed by each TLP. A transaction may be transmitted if thetransaction does not exceed a credit limit. Upon receiving a response anamount of credit is restored. One example of an advantage of such acredit scheme is that the latency of credit return does not affectperformance, provided that the credit limit is not encountered, amongother potential advantages.

In one embodiment, four transaction address spaces can include aconfiguration address space, a memory address space, an input/outputaddress space, and a message address space. Memory space transactionsinclude one or more of read requests and write requests to transfer datato/from a memory-mapped location. In one embodiment, memory spacetransactions are capable of using two different address formats, e.g., ashort address format, such as a 32-bit address, or a long addressformat, such as 64-bit address. Configuration space transactions can beused to access configuration space of various devices connected to theinterconnect. Transactions to the configuration space can include readrequests and write requests. Message space transactions (or, simplymessages) can also be defined to support in-band communication betweeninterconnect agents. Therefore, in one example embodiment, transactionlayer 205 can assemble packet header/payload 206.

A Link layer 210, also referred to as data link layer 210, can act as anintermediate stage between transaction layer 205 and the physical layer220. In one embodiment, a responsibility of the data link layer 210 isproviding a reliable mechanism for exchanging Transaction Layer Packets(TLPs) between two components on a link. One side of the Data Link Layer210 accepts TLPs assembled by the Transaction Layer 205, applies packetsequence identifier 211, i.e. an identification number or packet number,calculates and applies an error detection code, i.e. CRC 212, andsubmits the modified TLPs to the Physical Layer 220 for transmissionacross a physical to an external device.

In one example, physical layer 220 includes logical sub block 221 andelectrical sub-block 222 to physically transmit a packet to an externaldevice. Here, logical sub-block 221 is responsible for the “digital”functions of Physical Layer 221. In this regard, the logical sub-blockcan include a transmit section to prepare outgoing information fortransmission by physical sub-block 222, and a receiver section toidentify and prepare received information before passing it to the LinkLayer 210.

Physical block 222 includes a transmitter and a receiver. Thetransmitter is supplied by logical sub-block 221 with symbols, which thetransmitter serializes and transmits onto to an external device. Thereceiver is supplied with serialized symbols from an external device andtransforms the received signals into a bit-stream. The bit-stream isde-serialized and supplied to logical sub-block 221. In one exampleembodiment, an 8b/10b transmission code is employed, where ten-bitsymbols are transmitted/received. Here, special symbols are used toframe a packet with frames 223. In addition, in one example, thereceiver also provides a symbol clock recovered from the incoming serialstream.

As stated above, although transaction layer 205, link layer 210, andphysical layer 220 are discussed in reference to a specific embodimentof a protocol stack (such as a PCIe protocol stack), a layered protocolstack is not so limited. In fact, any layered protocol may beincluded/implemented and adopt features discussed herein. As an example,a port/interface that is represented as a layered protocol can include:(1) a first layer to assemble packets, i.e. a transaction layer; asecond layer to sequence packets, i.e. a link layer; and a third layerto transmit the packets, i.e. a physical layer. As a specific example, ahigh performance interconnect layered protocol, as described herein, isutilized.

Referring next to FIG. 3, an example embodiment of a serial point topoint fabric is illustrated. A serial point-to-point link can includeany transmission path for transmitting serial data. In the embodimentshown, a link can include two, low-voltage, differentially driven signalpairs: a transmit pair 306/311 and a receive pair 312/307. Accordingly,device 305 includes transmission logic 306 to transmit data to device310 and receiving logic 307 to receive data from device 310. In otherwords, two transmitting paths, i.e. paths 316 and 317, and two receivingpaths, i.e. paths 318 and 319, are included in some implementations of alink.

A transmission path refers to any path for transmitting data, such as atransmission line, a copper line, an optical line, a wirelesscommunication channel, an infrared communication link, or othercommunication path. A connection between two devices, such as device 305and device 310, is referred to as a link, such as link 315. A link maysupport one lane—each lane representing a set of differential signalpairs (one pair for transmission, one pair for reception). To scalebandwidth, a link may aggregate multiple lanes denoted by xN, where N isany supported link width, such as 1, 2, 4, 8, 12, 16, 32, 64, or wider.

A differential pair can refer to two transmission paths, such as lines316 and 317, to transmit differential signals. As an example, when line316 toggles from a low voltage level to a high voltage level, i.e. arising edge, line 317 drives from a high logic level to a low logiclevel, i.e. a falling edge. Differential signals potentially demonstratebetter electrical characteristics, such as better signal integrity, i.e.cross-coupling, voltage overshoot/undershoot, ringing, among otherexample advantages. This allows for a better timing window, whichenables faster transmission frequencies.

In one embodiment, a new High Performance Interconnect (HPI) isprovided. HPI can include a next-generation cache-coherent, link-basedinterconnect. As one example, HPI may be utilized in high performancecomputing platforms, such as workstations or servers, including insystems where PCIe or another interconnect protocol is typically used toconnect processors, accelerators, I/O devices, and the like. However,HPI is not so limited. Instead, HPI may be utilized in any of thesystems or platforms described herein. Furthermore, the individual ideasdeveloped may be applied to other interconnects and platforms, such asPCIe, MIPI, QPI, etc.

To support multiple devices, in one example implementation, HPI caninclude an Instruction Set Architecture (ISA) agnostic (i.e. HPI is ableto be implemented in multiple different devices). In another scenario,HPI may also be utilized to connect high performance I/O devices, notjust processors or accelerators. For example, a high performance PCIedevice may be coupled to HPI through an appropriate translation bridge(i.e. HPI to PCIe). Moreover, the HPI links may be utilized by many HPIbased devices, such as processors, in various ways (e.g. stars, rings,meshes, etc.). FIG. 4 illustrates example implementations of multiplepotential multi-socket configurations. A two-socket configuration 405,as depicted, can include two HPI links; however, in otherimplementations, one HPI link may be utilized. For larger topologies,any configuration may be utilized as long as an identifier (ID) isassignable and there is some form of virtual path, among otheradditional or substitute features. As shown, in one example, a foursocket configuration 410 has an HPI link from each processor to another.But in the eight socket implementation shown in configuration 415, notevery socket is directly connected to each other through an HPI link.However, if a virtual path or channel exists between the processors, theconfiguration is supported. A range of supported processors includes2-32 in a native domain. Higher numbers of processors may be reachedthrough use of multiple domains or other interconnects between nodecontrollers, among other examples.

The HPI architecture includes a definition of a layered protocolarchitecture, including in some examples, protocol layers (coherent,non-coherent, and, optionally, other memory based protocols), a routinglayer, a link layer, and a physical layer including associated I/Ologic. Furthermore, HPI can further include enhancements related topower managers (such as power control units (PCUs)), design for test anddebug (DFT), fault handling, registers, security, among other examples.FIG. 5 illustrates an embodiment of an example HPI layered protocolstack. In some implementations, at least some of the layers illustratedin FIG. 5 may be optional. Each layer deals with its own level ofgranularity or quantum of information (the protocol layer 505 a,b withpackets 530, link layer 510 a,b with flits 535, and physical layer 505a,b with phits 540). Note that a packet, in some embodiments, mayinclude partial flits, a single flit, or multiple flits based on theimplementation.

As a first example, a width of a phit 540 includes a 1 to 1 mapping oflink width to bits (e.g. 20 bit link width includes a phit of 20 bits,etc.). Flits may have a greater size, such as 184, 192, or 200 bits.Note that if phit 540 is 20 bits wide and the size of flit 535 is 184bits then it takes a fractional number of phits 540 to transmit one flit535 (e.g. 9.2 phits at 20 bits to transmit an 184 bit flit 535 or 9.6 at20 bits to transmit a 192 bit flit, among other examples). Note thatwidths of the fundamental link at the physical layer may vary. Forexample, the number of lanes per direction may include 2, 4, 6, 8, 10,12, 14, 16, 18, 20, 22, 24, etc. In one embodiment, link layer 510 a,bis capable of embedding multiple pieces of different transactions in asingle flit, and one or multiple headers (e.g. 1, 2, 3, 4) may beembedded within the flit. In one example, HPI splits the headers intocorresponding slots to enable multiple messages in the flit destined fordifferent nodes.

Physical layer 505 a,b, in one embodiment, can be responsible for thefast transfer of information on the physical medium (electrical oroptical etc.). The physical link can be point-to-point between two Linklayer entities, such as layer 505 a and 505 b. The Link layer 510 a,bcan abstract the Physical layer 505 a, b from the upper layers andprovides the capability to reliably transfer data (as well as requests)and manage flow control between two directly connected entities. TheLink Layer can also be responsible for virtualizing the physical channelinto multiple virtual channels and message classes. The Protocol layer520 a,b relies on the Link layer 510 a,b to map protocol messages intothe appropriate message classes and virtual channels before handing themto the Physical layer 505 a,b for transfer across the physical links.Link layer 510 a,b may support multiple messages, such as a request,snoop, response, writeback, non-coherent data, among other examples.

The Physical layer 505 a,b (or PHY) of HPI can be implemented above theelectrical layer (i.e. electrical conductors connecting two components)and below the link layer 510 a,b, as illustrated in FIG. 5. The Physicallayer and corresponding logic can reside on each agent and connects thelink layers on two agents (A and B) separated from each other (e.g. ondevices on either side of a link). The local and remote electricallayers are connected by physical media (e.g. wires, conductors, optical,etc.). The Physical layer 505 a,b, in one embodiment, has two majorphases, initialization and operation. During initialization, theconnection is opaque to the link layer and signaling may involve acombination of timed states and handshake events. During operation, theconnection is transparent to the link layer and signaling is at a speed,with all lanes operating together as a single link. During the operationphase, the Physical layer transports flits from agent A to agent B andfrom agent B to agent A. The connection is also referred to as a linkand abstracts some physical aspects including media, width and speedfrom the link layers while exchanging flits and control/status ofcurrent configuration (e.g. width) with the link layer. Theinitialization phase includes minor phases e.g. Polling, Configuration.The operation phase also includes minor phases (e.g. link powermanagement states).

In one embodiment, Link layer 510 a,b can be implemented so as toprovide reliable data transfer between two protocol or routing entities.The Link layer can abstract Physical layer 505 a,b from the Protocollayer 520 a,b, and can be responsible for the flow control between twoprotocol agents (A, B), and provide virtual channel services to theProtocol layer (Message Classes) and Routing layer (Virtual Networks).The interface between the Protocol layer 520 a,b and the Link Layer 510a,b can typically be at the packet level. In one embodiment, thesmallest transfer unit at the Link Layer is referred to as a flit whicha specified number of bits, such as 192 bits or some other denomination.The Link Layer 510 a,b relies on the Physical layer 505 a,b to frame thePhysical layer's 505 a,b unit of transfer (phit) into the Link Layer's510 a,b unit of transfer (flit). In addition, the Link Layer 510 a,b maybe logically broken into two parts, a sender and a receiver. Asender/receiver pair on one entity may be connected to a receiver/senderpair on another entity. Flow Control is often performed on both a flitand a packet basis. Error detection and correction is also potentiallyperformed on a flit level basis.

In one embodiment, Routing layer 515 a,b can provide a flexible anddistributed method to route HPI transactions from a source to adestination. The scheme is flexible since routing algorithms formultiple topologies may be specified through programmable routing tablesat each router (the programming in one embodiment is performed byfirmware, software, or a combination thereof). The routing functionalitymay be distributed; the routing may be done through a series of routingsteps, with each routing step being defined through a lookup of a tableat either the source, intermediate, or destination routers. The lookupat a source may be used to inject a HPI packet into the HPI fabric. Thelookup at an intermediate router may be used to route an HPI packet froman input port to an output port. The lookup at a destination port may beused to target the destination HPI protocol agent. Note that the Routinglayer, in some implementations, can be thin since the routing tables,and, hence the routing algorithms, are not specifically defined byspecification. This allows for flexibility and a variety of usagemodels, including flexible platform architectural topologies to bedefined by the system implementation. The Routing layer 515 a,b relieson the Link layer 510 a,b for providing the use of up to three (or more)virtual networks (VNs)—in one example, two deadlock-free VNs, VN0 andVN1 with several message classes defined in each virtual network. Ashared adaptive virtual network (VNA) may be defined in the Link layer,but this adaptive network may not be exposed directly in routingconcepts, since each message class and virtual network may havededicated resources and guaranteed forward progress, among otherfeatures and examples.

In one embodiment, HPI can include a Coherence Protocol layer 520 a,b issupport agents caching lines of data from memory. An agent wishing tocache memory data may use the coherence protocol to read the line ofdata to load into its cache. An agent wishing to modify a line of datain its cache may use the coherence protocol to acquire ownership of theline before modifying the data. After modifying a line, an agent mayfollow protocol requirements of keeping it in its cache until it eitherwrites the line back to memory or includes the line in a response to anexternal request. Lastly, an agent may fulfill external requests toinvalidate a line in its cache. The protocol ensures coherency of thedata by dictating the rules all caching agents may follow. It alsoprovides the means for agents without caches to coherently read andwrite memory data.

Two conditions may be enforced to support transactions utilizing the HPICoherence Protocol. First, the protocol can maintain data consistency,as an example, on a per-address basis, among data in agents' caches andbetween those data and the data in memory. Informally, data consistencymay refer to each valid line of data in an agent's cache representing amost up-to-date value of the data and data transmitted in a coherenceprotocol packet can represent the most up-to-date value of the data atthe time it was sent. When no valid copy of the data exists in caches orin transmission, the protocol may ensure the most up-to-date value ofthe data resides in memory. Second, the protocol can providewell-defined commitment points for requests. Commitment points for readsmay indicate when the data is usable; and for writes they may indicatewhen the written data is globally observable and will be loaded bysubsequent reads. The protocol may support these commitment points forboth cacheable and uncacheable (UC) requests in the coherent memoryspace.

The HPI Coherence Protocol also may ensure the forward progress ofcoherence requests made by an agent to an address in the coherent memoryspace. Certainly, transactions may eventually be satisfied and retiredfor proper system operation. The HPI Coherence Protocol, in someembodiments, may have no notion of retry for resolving resourceallocation conflicts. Thus, the protocol itself may be defined tocontain no circular resource dependencies, and implementations may takecare in their designs not to introduce dependencies that can result indeadlocks. Additionally, the protocol may indicate where designs areable to provide fair access to protocol resources.

Logically, the HPI Coherence Protocol, in one embodiment, can includethree items: coherence (or caching) agents, home agents, and the HPIinterconnect fabric connecting the agents. Coherence agents and homeagents can work together to achieve data consistency by exchangingmessages over the interconnect. The link layer 510 a,b and its relateddescription can provide the details of the interconnect fabric includinghow it adheres to the coherence protocol's requirements, discussedherein. (It may be noted that the division into coherence agents andhome agents is for clarity. A design may contain multiple agents of bothtypes within a socket or even combine agents behaviors into a singledesign unit, among other examples.)

In some implementations, HPI can utilize an embedded clock. A clocksignal can be embedded in data transmitted using the interconnect. Withthe clock signal embedded in the data, distinct and dedicated clocklanes can be omitted. This can be useful, for instance, as it can allowmore pins of a device to be dedicated to data transfer, particularly insystems where space for pins is at a premium.

The Link layer can guarantee reliable data transfer between two protocolor routing entities. The Link layer can abstract the Physical layer fromthe Protocol layer, handle flow control between two protocol agents, andprovide virtual channel services to the Protocol layer (Message Classes)and Routing layer (Virtual Networks).

In some implementations, the Link layer can deal with a fixed quantum ofinformation, termed a flit. In one example, the flit can be defined tobe 192 bits in length. However, any range of bits, such as 81-256 (ormore) may be utilized in different variations. A large flit size, suchas 192 bits, may include format, cyclic redundancy check (CRC), andother changes. For instance, a larger flit length can also permit theCRC field to be expanded (e.g., to 16 bits) to handle the larger flitpayload. The number of phits or unit intervals (UI) (e.g., the time usedto transfer a single bit or phit, etc.) to transfer a single flit canvary with link width. For instance, a 20 lane or bit link width cantransfer a single 192 bit flit in 9.6 UI, while an 8 lane link widthtransfers the same flit in 24 UI, among other potential examples. Thelink layer crediting and protocol packetizing can also be based on aflit.

FIG. 6 illustrates a representation 600 of a generalized flit for an 8lane link width. Each column of the representation 600 can symbolize alink lane and each row a respective UI. In some implementations, asingle flit can be subdivided into two or more slots. Distinct messagesor link layer headers can be included in each slot, allowing multipledistinct, and in some cases, independent messages corresponding topotentially different transactions to be sent in a single flit. Further,the multiple messages included in slots of a single flit may also bedestined to different destination nodes, among other examples. Forinstance, the example of FIG. 6 illustrates a flit format with threeslots. The shaded portions can represent the portion of the flitincluded in a respective slot.

In the example of FIG. 6, three slots, Slots 0, 1, and 2, are provided.Slot 0 can be provided 72 bits of flit space, of which 22 bits arededicated to message header fields and 50 bits to message payload space.Slot 1 can be provided with 70 bits of flit space, of which 20 bits arededicated to message header fields and 50 bits to message payload space.The difference in message header field space between can be optimized toprovide that certain message types will be designated for inclusion inSlot 0 (e.g., where more message header encoding is utilized). A thirdslot, Slot 2, can be provided that occupies substantially less spacethan Slots 0 and 1, in this case utilizing 18 bits of flit space. Slot 2can be optimized to handle those messages, such as acknowledges, creditreturns, and the like that do no utilize larger message payloads.Additionally, a floating payload field can be provided that allows anadditional 11 bits to be alternatively applied to supplement the payloadfield of either Slot 0 or Slot 1.

Continuing with the specific example of FIG. 6, other fields can beglobal to a flit (i.e., apply across the flit and not to a particularslot). For instance, a header bit can be provided together with a 4-bitflit control field that can be used to designate such information as avirtual network of the flit, identify how the flit is to be encoded,among other examples. Additionally, error control functionality can beprovided, such as through a 16-bit cyclic CRC field, among otherpotential examples.

A flit format can be defined so as to optimize throughput of messages onthe Link layer. Some traditional protocols have utilized unslotted,smaller flits. For instance, in QPI an 80-bit flit was utilized. Whilethe flit throughput of a larger (e.g., 192-bit flit) may be lower,message or packet throughput can be increased by optimizing use of theflit data. For instance, in some implementations of QPI, the entire80-bit flit space was utilized regardless of the message size or type.By subdividing a larger flit into slots of predetermined lengths andfields, the 192 flit length can be optimized realizing higher efficiencyeven in instances when one or more of the available slots are sometimesunused. Indeed, Link layer traffic can be assumed to include manydifferent types of messages and traffic, including messages and packetswith varying header lengths and fields. The respective lengths andorganization of slots defined in a flit can be defined so as tocorrespond with the statistical or expected frequency of variousmessages and the needs of these messages. For instance, two larger slotscan be defined for every small slot, to accommodate an expectedstatistical frequency of messaging using these larger message types andheader lengths, among other example. Further, flexibility can also beprovided to further accommodate the varied traffic, such as through afloating payload field, as in the example of FIG. 6. In some instances,a flit format can be fixed, including the bits dedicated to particularslots in the flit.

In the example of FIG. 6, a “Hdr” field can be provided for the flitgenerally and represent a header indication for the flit. In someinstances, the Hdr field can indicate whether the flit is a header flitor a data flit. In data flits, the flit can still remain slotted, butomit or replace the use of certain fields with payload data. In somecases, data fields may include an opcode and payload data. In the caseof header flits, a variety of header fields can be provided. In theexample of FIG. 6, “Oc” fields can be provided for each slot, the Ocfield representing an opcode. Similarly, one or more slots can have acorresponding “msg” field representing a message type of thecorresponding packet to be included in the slot, provided the slot isdesigned to handle such packet types, etc. “DNID” fields can represent aDestination Node ID, a “TID” field can represent a transaction ID, a“RHTID” field can represent either a requestor node ID or a home trackerID, among other potential fields. Further, one or more slots can beprovided with payload fields. Additionally, a CRC field can be includedwithin a flit to provide a CRC value for the flit, among other examples.

In some implementations, link width can vary during the life of thelink. For instance, the Physical layer can transition between link widthstates, such as to and from a full or original lane width and adifferent or partial lane width. For example, in some implementations, alink can be initialized to transfer data over 20 lanes. Later, the linkcan transition to a partial width transmitting state where only 8 lanesare actively used, among many other potential examples. Such lane widthtransitions can be utilized, for instance, in connection with powermanagement tasks governed by one or more power control units (PCU) amongother examples.

As noted above, link width can influence flit throughput rate. FIG. 7 isa representation of an example 192-bit flit sent over an 8 lane link,resulting in throughput of the flit at 24 UI. Further, as shown in theexample of FIG. 7, bits of the flit can be sent out of order in someinstances, for example, to send more time-sensitive fields earlier inthe transfer (e.g., flit type fields (e.g., data or header flit),opcodes, etc.), preserve or facilitate particular error detection orother functionality embodied in the flit, among other examples. Forinstance, in the example of FIG. 7, bits 191, 167, 143, 119, 95, 71, 47,and 23 are sent in parallel on lanes L7 through L0 during a first UI(i.e., UI0) of transfer, while bits 168, 144, 120, 96, 72, 48, 24, and 0are sent during the 24^(th) (or final) UI of the flit transfer (i.e.,UI23). It should be appreciated that other ordering schemes, flitlengths, lane widths, etc. can be utilized in other implementations andexamples.

In some instances, the length of the flit can be a multiple of thenumber of active lanes. In such instances, the flit can be transmittedevenly on all active lanes and transfer of the flit can endsubstantially simultaneously at a clean (i.e., non-overlapping)boundary. For example, as shown in the representation of FIG. 8, bits ofa flit can be considered to be transmitted in consecutive groupings of 4bits, or “nibbles.” In this example, a 192 bit flit is to be transferredover an 8 lane link. As 192 is a multiple of 8, the entire flit can becleanly transferred over the 8 lane link in 24 UI. In other instances,the flit width may not be a multiple of the number of active lanes. Forinstance, FIG. 9 shows another representation of an example 192 bittransferred over 20 lanes. As 192 is not evenly divisible by 20,transfer of the fill flit would require a non-integer number ofintervals (e.g., 9.6 UI). In such cases, rather than wasting “extra”lanes not utilized during the 10th UI of transfer, a second overlappingflit can be transferred with the final bits of a preceding flit. Suchoverlapping, or swizzling, of the flits can result in jagged flitboundaries and flit bits sent out of order in some implementations. Thepattern utilized for the transfer can be configured to allow moretime-sensitive fields of the flit to be transferred earlier in the flit,preservation of error detection and correction, among otherconsiderations. Logic can be provided in one or both of the Physical andLink layers to transfer flit bits according to such patterns anddynamically change between patterns based on the current link width.Further logic can be provided to re-order and re-construct flits fromsuch swizzled or ordered bit streams, among other examples.

In some implementations, flits can be characterized as header flits(e.g., bearing packet header data) or data flits (e.g., bearing packetpayload data). Returning to FIG. 6, a flit format can be defined thatincludes three (3) distinct slots (e.g., 0, 1, and 2), allowing up tothree headers to be transferred in a single flit (e.g., one header ineach slot). Accordingly, each slot can have both control fields and apayload field. In addition to these, payload fields can be defined foreach header (and slot). Further, a floating payload field can be definedthat can be flexibly used as extra payload length for two or more of theslots (e.g., by either slot 0 or slot 1), based on the header types inthese slots. The floating field can enable, in one implementation, 11extra bits of payload for either Slot 0 or Slot 1. Note inimplementations defining a larger flit more floating bits may be usedand in smaller flits less floating bits may be provided.

In some implementations, by allowing a field to float between the twoslots, extra bits can be provided as needed for certain messages whilestill staying within a predefined flit length (e.g., 192 bits) andmaximizing the utilization of the bandwidth. Turning to the examples ofFIG. 10, two instances 1005, 1010 of an example 192-bit flit are shownon an 8 lane data link. In one instance, a flit (e.g., 1005) can includethree slots, Slots 0, 1, and 2. Each of Slots 0 and 1 can include 50-bitpayload fields. The floating field can be provided to alternativelyextend the payload field of the either Slot 0 or Slot 1 by the fieldlength (e.g., 11 bits) of the floating field. The use of a floatingfield can further extend the efficiency gains provided through adefined, multi-slot flit format. The sizing of the slots within theflit, and the types of messages that can be placed in each slot, canpotentially provide increased bandwidth even with a reduced flit rate.

In the particular example of FIG. 6, the messages that can use Slots 1and 2 can be optimized, reducing the number of bits to be set aside toencode these slots' opcodes. When a header having more bits that Slot 0can provide enters the Link layer, slotting algorithms can be providedto allow it to take over Slot 1 payload bits for additional space.Special control (e.g. LLCTRL) flits may also be provided that consumeall three slots worth of bits for their needs. Slotting algorithms mayalso exist to allow individual slots to be utilized while other slotscarry no information, for cases where the link is partially busy.

In the particular example of FIG. 10, example use of a floating flitfield is shown. For instance, in the case of Standard Address Snoop(SA-S) Headers, only a single SA-S message (and header) may be permittedto be sent in the same flit (e.g., to prevent conflicts or where theSA-S payload utilizes a larger than 50-bit payload, etc.). Consequently,in such examples, a SA-S may only be sent in either Slot 0 or Slot 1 ofthe same flit in such instances. In the example of flit 1005, an SA-Sheader is included in Slot 0 and is to make use of the floating field.Consequently, in the example of flit 1005, the use of the floating fieldis dedicated to extend the payload of Slot 0's payload. In anotherexample, of flit 1010, the SA-S header is to occupy Slot 1. In theexample of flit 1010, the floating field is instead dedicated to extendthe payload of Slot 1. Other potential examples can also make use of theflexibility provided through a floating payload field of a slotted flitutilizing principles illustrated in the particular examples of FIGS. 6and 10.

In one embodiment, such as that illustrated in connection with FIG. 6,two slots, Slot 0 and 1, can be defined as having equally sized payloadfields, while Slot 2 has a much smaller payload field for use by aparticular subset of headers that lack the use of such larger payloadfields, for instance. Further, in one example, Slot 1 and 2 controlfields may not carry full Message Class encodings (unlike Slot 0), andSlot 2 may not carry a full opcode encoding, among other potentialimplementations.

As noted above, in some implementations, Slots 1 and 2 may not carryfull Message Class encodings, as not all bits are utilized due toslotting restrictions. Slot 1 can carries a Message Class bit 0. Here,request (REQ) and snoop (SNP) packets are allowed. In thisimplementation, REQ and SNP Message Class encodings are differentiatedby bit 0. As a result, if a designer wanted to allow different messageclasses in partial message class field, they could either select adifferent bit position (i.e. an upper bit that differentiates twodifferent types of messages) or assign different message types to thelower order bit. However, here the upper two bits are implied as 0'swith the lower bit distinguishing between a REQ and a SNP. In thisexample, Slot 2 carries no Message Class bits, as only response (RSP)(encoding 2) packets are allowed in. Therefore, the Message Classencoding for Slot 2 is a RSP-2. Slot 2 can also carry a partial opcode.As above, one or more of the opcode bits can be assumed to be 0. As aresult, partial message class fields and partial operation code fieldsmay be utilized that define a subset of messages and op codes that maybe utilized. Note that multiple sets of opcodes and messages classes maybe defined. Here, if a lower order bit of the message class is used,then a subset of message types (i.e. MSG type 1/MSG type 2) isavailable. However, if 2 bits are used, then a larger subset is provided(e.g. Message Type 1/Message Type 2/Message Type 3/Message Type 4),among other examples.

Message class encodings can correspond to particular header types to beincluded (or to utilize) one or more defined slots in a flit. Forinstance, a header may have multiple sizes. In one example, a three slotflit can be defined to support potentially four sizes of header, basedon header type. Table 1 includes an exemplary listing of potentialheader formats and associated sizes:

TABLE 1 Header Format Header Size Description SA Single Slot RequestSA-S Single Slot Snoops (incorporates floating payload field) SA-DSingle Slot Data header SR-U Small Slot Completion without data SR-OSingle Slot Ordering SR-C Single Slot Conflict resolution SR-D SingleSlot Data header PW Dual Slot Partial write PR Dual Slot Partial readP2P Dual Slot Peer-to-peer NCM Dual Slot Non-coherent messagingSlot-NULL Single Slot (or Control flit Opcode only) LLCRD Small SlotControl flit LLCTRL Full Flit Control flit

Small (or single) slot headers can be for those message small enough tofit in Slot 2, and that don't have protocol ordering requirementsforcing them into Slot 0. A small slot header can also be placed in Slot0, if the slotting restrictions for the flit call for it. The singleslot header can be for those messages with payload that can fit in Slot0 or Slot 1. Some single slot headers may also make use of the floatingpayload field. For instance, Standard Address Snoop (SA-S) Headers, inone embodiment, may not be sent in both slot 0 and slot 1 of the sameflit in the example where only one HTID or floating field exists.Certain single slot headers may use Slot 0 based on protocol orderingrequirements. The dual slot header can be for those messages largeenough that they are to consume both the Slot 0 and Slot 1 payloadfields, in addition to the floating payload field, among other examples.

A slot NULL opcode may include a special opcode that can be used, in oneexample, in either Slot 0 or Slot 1. For Slot 0, Slot_NULL may be usedwhen the link layer has no header to transmit in Slot 0, but it doeshave a header to transmit in Slot 1 or 2. When Slot_NULL is used in Slot0, the Slot 0 payload is considered reserved (RSVD), among otherexamples. In some implementations, Slot_NULL can be utilized in Slot 1potentially under two conditions. First, when Slot 0 is encoding a dualslot or special control header, and thus consuming the Slot 1 payload.In such instances, the Slot 1 opcode can be set to Slot_NULL. The secondcondition is when the link layer has nothing to send in Slot 1, but doeshave a valid Single Slot header for Slot 0 or Small Slot Header for Slot2. Under this condition, the Slot 1 opcode can be set to Slot_NULL andthe Slot 1 payload can be considered Reserved, among other potentialexamples.

In some implementations, the small Slot 2, may include a reduced numberof opcode bits. When the link layer has nothing to send in Slot 2, itmay send an “Implicit NULL” by encoding a specific opcode, such as alink layer credit opcode and setting the Slot 2 payload field to allzeros. The receiver of this Slot 2 encoding can process it as a linklayer credit message (except in the case of the special control flits),but the all zeros encoding will have no effect on the Credit andAcknowledge state. In the case of special control flits, because theycan consume the entire flit, the Slot 2 payload can be considered RSVDand the Implicit NULL will be ignored. Where the link layer has nothingto send in any of the three slots and the CRD/ACK fields, the link layermay transmit a special control null message, among other examples.

Slotting restrictions can be defined for one or more of the definedslots of a flit. In one embodiment, dual slot headers may only havetheir Message Class and Opcode placed in Slot 0. When Slot 0 contains aDual Slot Header, Slot 1 may encode a Slot_NULL opcode, as the Slot 1Payload field will be consumed by the Slot 0 header. When Slot 0contains a Slot_NULL, single slot, or small slot header, Slots 1 and 2may both encode a non-NULL header. Only small slot headers are allowedin Slot 2 in this particular example (e.g., illustrated in FIG. 6). Whenboth Slot 0 and Slot 1 contain single slot headers, one may be of a typethat consumes the floating payload field. If neither Slot 0 or Slot 1contain a header type that consumes the floating payload field, thefield may be considered RSVD.

Additionally, in some implementations, the Link layer can utilizemultiple different types of virtual network or virtual channel credits.In one example, pooled virtual network adaptive (VNA) credits can besupported and a VNA field can be provided. In one exampleimplementation, when the VNA field indicates a non-VNA flit (e.g., aflit that utilizes a different credit pool), the header may bedesignated to be placed in Slot 0. Further, the Slot 2 opcode mayinclude a Slot_2 credit in this case. Further, when Slot 0 encodes aspecial control Header, both Slot 1 and Slot 2 control fields may be setto fixed values, and no headers may be placed in these slots, amongother potential implementations.

As noted above, in header flits, a variety of different fields can beprovided to be incorporated in corresponding flit slots, such asillustrated in the particular example of FIG. 6. Note that the fieldsillustrated and described a provided by way of example and additional orsubstitute fields can also be incorporated. Indeed, some of the fieldsdescribed may be optional and be omitted in some implementations, amongother examples.

In one example, a message class (MC) field can be provided, as well asother fields. In some examples, the Protocol layer can use the MessageClass field to define the Protocol Class which also acts as the MajorOpcode field. The Link layer can use the Message Class field as part ofthe virtual channel (VC) definition. Some Protocol Classes/VC can usemultiple Message Class encodings due to the number of opcodes that areto be encoded, among other examples. For instance, Requests (REQ),Snoops (SNP), Response (RSP), writeback, non-coherent bypass, andnon-coherent standard types can be supported. If each type encodedsixteen operations, then there would be an opcode space of 96operations. And if another mode bit or other opcode space was definedfor each type, then another 96 operations could be provided; and so on.

In one example, an Opcode field can additionally be provided. TheProtocol layer may use the opcode in conjunction with the Message Classto form a complete opcode (i.e. define the message class type and theoperation within). As an example, the same opcode with a REQ messagetype may define a first request operation, while the same opcode with aSNP message class may define a second, different SNP operation, amongother examples. The Link Layer may use the opcode to distinguish, forinstance, between a Home Agent target or a Caching Agent target forpackets when a Home Agent and a Caching Agent share the same NodeID.Additionally, the Link Layer may also use the opcode to determine packetsize, among other potential uses.

As noted above, flit headers can further include a Virtual networkAdaptive (VNA) field. In one example, when a VNA field is set to a firstvalue, the field can indicate that the flit is using VNA credits. Whenset to a second value, the flit is using VN0 or VN1 credits, among otherpotential implementations. In one embodiment, a value may indicate theflit is a single slot flit and slots 1 and 2 codes can be defined asNULL.

A Virtual Network (VN) field can also be provided and indicate for aflit if the header(s) in the flit are utilizing a particular virtualnetwork, such as a virtual network VN0 or VN1. This may be used for bothcrediting purposes and to indicate which virtual network a messageshould drain to if using VNA. If one VN bit is provided for the entireflit, any VNA flit that contains multiple headers can ensure that all ofthem are draining to VN0 or all of them are draining to VN1.Alternatively, multiple VN bits may be provided. For non VNA flits, onlySlot 0 may be allowed to have a non-control opcode, so the VN mayindicate that header's network.

In some implementations, slots in a flit can be used for small payloadmessages such as credit returns, ACKs, NAKs, among others. In oneexample, a channel field can be provided that can be encoded for use incredit returns. This encoding, in combination with the Virtual Networkfield, may provide the Virtual Channel that a credit return maps to.Where a Message Class has multiple encodings, they may all map to asingle Channel value for crediting. When the credit return type is VNA,the Channel value can be ignored. Use of RSVD encodings may be treatedas an error by the receiving component. Table 2 includes examples ofdifferent Channel options that can be encoded. Note that any combinationof bits (or bits representing a hexidecimal value) may be utilized. Asan example, a lower order of 3 bits can be used for encoding.

TABLE 2 Channel REQ: Request SNP: Snoop RSP: Response RSVD: Reserved WB:Write back NCB: Non-coherent bypass NCS: Non-coherent standard

Acknowledgement, or ACK, fields can also be provided as header fields tobe included in a flit slot. An ACK field may be used by the Link layerto communicate from a receiver to a sender error free receipt of flits.ACK having a first value indicates that a number of flits, such as 4, 8,or 12, have been received without error. When a sender receives an ACKit may deallocate the corresponding flits from the Link Layer RetryQueue. Ack and Ack fields can be used in credit return control flits(e.g., LLCRD), with the total number of Acknowledges being returneddetermined by creating the full acknowledge return value (Acknowledgefirst portion, ACK, Acknowledge second portion), among other examples.

As noted above, a Header indication bit (Hdr) can also be provided insome implementations and can be used for one or more purposes. Forinstance, a Hdr packet can identify whether the packet is a header ordata flit, can indicate that the flit is the start of a new packet, aswell as indicate the start of an interleaved Link Layer Control flit.The Hdr can be set for the first flit of all packets. Further, anAddress field can be provided to identify a global system address. Allcoherent transactions may be a number of byte aligned and may return thenumber of bytes of data, eliminating the need for some portion of theAddress bits (e.g. at 64 bytes, the lower 6 bits may be omitted). Forcertain other packets, a full byte level address is to be utilized. ALength field can be provided in some examples to indicate a length ofthe requested data in bytes for any transaction that is doing a partialread. The partial read specifies the offset (e.g. the lower portion ofthe address bits omitted above) and the Length. Valid lengths are 0 tothe number of bytes that the transactions are aligned to less one, amongother examples.

Additional fields can be included. A Byte Enable field can be providedin some instances to indicate the valid bytes for any transaction doinga partial write. A Byte Enable field may have any number 0 to the numberof bytes that the transactions are aligned to less one. A Request TID(RTID) field can be used to uniquely identify the different requestsfrom a single Protocol Agent. A Home tracker ID (HTID) field can be usedin Snoop packets and Snoop Response packets to indicate the Home TrackerID of the transaction the snoop and its response are to be associatedwith. An RHTID field can also be provided in some implementations andflexibly embody an RTID or an HTID, depending on the opcode. Forinstance, for a snoop, RHTID can be interpreted as RTID, as snoops havean explicit HTID field. For response packets, on the other hand,targeting a home agent, RHTID can be interpreted as HTID. Additionally,for response packets targeting a cache agent, RHTID can be interpretedas RTID for opcodes except FwdCnfltO, among other examples. In someimplementations, other message types can default to being interpreted asRTID.

In some implementations, additional fields can be provided such as aDestination Node ID (DNID) field, Requestor Node ID (RNID) field,Conflict Node ID (CNID) field, and Source Node ID (SNID) field. The DNIDcan identify the destination of a packet. It can be supplied by theProtocol Layer and used by the Link and Routing layers to guide packetsto their destinations. The RNID field can identify the originalrequester/initiator of a transaction and can be supplied by the ProtocolLayer. The CNID can be used in RspCnflt packets to indicate the node ID(NID) of the agent where the snoop experienced a conflict and theFwdCnfltO should be sent. A SNID field can be used in SR-D packets toindicate the Node ID of the agent transmitting the SR-D packet.

Additionally, a Prior Cache Line State (PCLS) field can be provided toencode the state of the cache line where it was found at either a peercaching agent or in a home node. For example, if the cache line wassupplied by a peer node in the F state, the field should be set to afirst value. If the cache line was sent by a home node, the home nodeshould set the field to reflect either the I state or S state dependingon the snoop responses it received. If an agent does not support thisfield it should always be encoded as a default value. Note the PCLSfield may be used for performance monitoring/tuning. A Non-CoherentProtected field can denote whether a request is to Normal or Protectedspace. See the table below for the encodings.

In some implementations, HPI Link layer can support a multi-slot flitwith explicit fields, such as those described above, as well as implicitfields. For instance, slot message encoding and opcodes can be regardedas implicit. For instance, Slots 1 and 2 may not carry full MessageClass encodings, as not all bits are required, in some instances, due toslotting restrictions. Slot 1 carries only Message Class bit 0, and onlyREQ and SNP packets may be allowed in this slot. REQ and SNP MessageClass encodings can be differentiated by bit 0, and the upper two bitscan be implied as 0's. Slot 2 may not carry Message Class hits, as onlyRSP (encoding 2) packets are allowed in this Slot. Therefore the MessageClass encoding for Slot 2 may be RSP-2. Slot 2 can also only carry aportion of an opcode, with a second portion of an opcode being assumedto be a default value. This means that RSP-2 packets with the secondportion holding the default value are allowed in Slot 2. Further, theComplete opcode field, in one embodiment, can be created by combiningthe full Message Class with the full Opcode field, forming a CompleteOpcode.

Additional examples of implicit fields can include packet length, whichcan be implied by the opcode. Further, the globally Unique TransactionID (UTID) may be formed by combining Requester NodeID with RequesterTransaction ID. Note, that there may be an overlap in the RTID spacebetween P2P and non-P2P transactions. For instance, the globally P2PUnique Transaction ID (P2PUTID) may be formed by combining RequesterNodeID with Requester Transaction ID.

In some implementations, such as that illustrated in the examples ofFIG. 6, the structure of the flit can permit Transaction IDs (TIDs) thatutilize 11 bits of flit space. As a result, pre-allocation and theenabling of distributed home agents may be removed. Furthermore, use of11 bits, in some implementations, allows for the TID to be used withouthaving use for an extended TID mode.

Link layer logic can be provided on each agent on each side of a link. Atransmitter of an agent or device can receive data from higher layers(e.g., a Protocol or Routing layer) and generate one or more flits totransfer the data to a receiver of a remote agent. The agent cangenerate a flit with two or more slots. In some instances, the agent canattempt to combine multiple messages or packets within a single flitutilizing the defined slots.

Link layer logic can include, in some implementations, dedicated pathscorresponding to each defined slot. The paths can be embodied in eitheror both hardware and software. A receiver of an agent can receive a flit(as re-constructed using the Physical layer) and Link layer logic canidentify each of the slots and process the slots using each slot'srespective path. The Link layer can process the flits, and the dataincluded in each slot, according to one or more encoded fields of theflit, such as a control field, header field, CRC field, etc.

In one illustrative example, a transmitter can receiver a write requestassociated with a first transaction, a snoop request associated withanother second transaction, and one or more acknowledges or creditreturns that it can send to (or through) another device. The transmittercan send a single flit to the other device over a serial data link of aninterconnect, the single flit to include headers for each of the writerequest, the snoop, and an acknowledge (e.g., a completion), each headeroccupying a respective flit slot (such as in the 3-slot implementationillustrated in the example of FIG. 6). The transmitter can buffer datait receives and identify opportunities to send multiple messages in asingle flit. The receiver can receive the compiled flit and process eachslot in parallel to identify and process each of the three messages,among many other potential examples.

In some implementations, multiple headers can be included in amulti-slot flit so as to send multiple messages using a single flit. Insome examples, the respective headers can each relate to fullyindependent transactions. In some implementations, the flexibility ofthe flit can be constrained such that each flit only contains messagesdirected to a particular virtual network. Other implementations mayabstain from such a condition.

In instances where slot messages are to all apply to a common one of aplurality of virtual networks, bits that would have traditionally beenreserved for identification of a slot's respective virtual network canbe dedicated to other uses, that in some implementations, furtherincreases efficiency gains introduced by the flit format, amongpotentially other benefits. In one example, all slots in a multi-slotheader flit may be aligned to a single virtual network such as only VNA,only VN0, or only VN1, etc. By enforcing this, per slot bits indicatingvirtual network can be removed. This increases the efficiency of flitbit utilization and potentially enables such other features, asexpanding from 10 bit TIDs to 11 bit TIDs, among other examples.Expanding to an 11 bit TID can, in some implementations, allow for theTID to be used without having use for an extended TID mode.

In HPI, a large CRC baseline can be used to provide error detection on alarger multi-slot flit. In some cases, the CRC baseline can even improveerror detection over traditional error detection, including other CRC,implementation. In one example, as shown in the example multi-slot flitof FIG. 6, 16 bits can be dedicated per flit to CRC. As a result of thelarger CRC, a larger payload may also be utilized. The 16 bits of CRC incombination with a polynomial used with those bits improves errordetection.

The value of a CRC field of a flit can be generated from a bit data maskrepresenting the payload of the flit. The CRC value can be generatedbased on a particular polynomial. In one example, such as the example ofFIG. 6, a 192 bit flit can include a 16 bit CRC field. Accordingly, a176 (non-CRC) bit data masks can be used with an XOR tree (based on theselected polynomial) to produce the 16 CRC bits. Note that the flitpayload bits can map vertically across UI within lanes. This maymaintain burst error protection.

Link layer logic of an agent can be used to generate the CRC value for aflit. The generated CRC value can be encoded in the CRC field of itscorresponding flit. The flit can then be sent over a serial data link toa receiver. The Link layer logic of the receiver can apply the samepolynomial used to generate the CRC value to the CRC value identified inthe CRC field of a received flit. The receiver can generate a checksumfrom the CRC value and compare the result against the remaining, non-CRCflit data to determine whether any bit errors resulted from thetransmission of the flit over the link. If an error exists on a lane,the checksum should produce a mismatched result, indicating one or morebit errors, among other examples. Additionally, in some implementations,the CRC code may be inverted after generation at the transmitter andinverted again before checking at the receiver, for instance, to preventa flit of potentially all 0's or all 1's from passing the CRC check.

The accuracy of a CRC can be based on the length of the CRC value andthe number of lanes utilized to send the flit. For instance, thepotential error burst rate can increase as the number of lanes used inthe link decreases. This can introduce additional complexity in HPIsystems supporting partial width transmitting states, for instance.

In some cases, the CRC polynomial can be designed based on the maximumtotal length of the block to be protected (data+CRC bits), the desirederror protection features, and the type of resources for implementingthe CRC, as well as the desired performance. In some examples, a CRCpolynomial can be derived from either an irreducible polynomial or anirreducible polynomial times the factor to detect all errors affectingan odd number of bits. However, in some instances, choosing a reduciblepolynomial can result in missed errors, due to the rings having zerodivisors, etc.

In one example implementation, a primitive polynomial can be utilized asthe generator for a CRC code to provide a resulting CRC code withmaximal total block length. For instance, if r is the degree of theprimitive generator polynomial, then the maximum block length can be(2^(r)−1), and the associated code can be able to detect any single-bitor double-bit errors. In another implementations, a generator polynomialg(x)=p(x)(1+x) can be utilized, where p(x) is a primitive polynomial ofdegree (r−1), a maximum block length is (2^(r-1)−1), and the resultingcode able to detect single, double, and triple errors, among otherexamples.

A polynomial g(x) that admits other factorizations may be utilized so asto balance the maximal total blocklength with a desired error detectionpower. For instance, BCH codes are a powerful class of such polynomials.Regardless of the reducibility properties of a generator polynomial ofdegree r, if it includes the “+1” term, the code can be able to detecterror patterns that are confined to a window of r contiguous bits. Thesepatterns can be referred to as “error bursts”. Such error bursts canresult, for instance, when an error affects one of the lanes of a link.

In one particular example, a 192 bit flit can include a 16 bit CRCfield. A 16 bit CRC polynomial can be implemented in Link layer logic togenerate values of the CRC field. In one embodiment, the polynomial canpermit detection of 1-bit, 2-bit, 3-bit, and 4-bit errors, detection oferrors of burst length 16 or less, with only 1:2¹⁶ of all other errorconditions going undetected. In one particular example, the 16 bit CRCpolynomial utilized can be 0x1b7db(x¹⁶+x¹⁵+x¹³+x¹²+x¹⁰+x⁹+x⁸+x7+x⁶+x⁴+x³+x¹+1) to provide an XOR depth of93, 4 bit random error detection, and 16 bit burst protection, amongother potential implementations and alternatives.

As noted above, the error detection properties of a CRC can be based onthe length of the CRC. For instance, in the case of a 16 bit CRCprotecting a 192 bit flit, error detection can capture errors of burstlength 16 or less. Such an implementation can effectively capturesubstantially all single-lane errors that could appear on a linkemploying 12 or more lanes to transmit the flit. However, for links orlink states utilizing fewer lanes to transmit the flit, a 16 bit CRC caninsufficient. For instance, a malfunction or error on a single lane ofan 8 lane link can result in errors with burst lengths as high as 24bits.

In some implementations, rolling CRC can be employed to extend the errordetection properties provided through a flit format dedicating a fixednumber of bits to a CRC. In one embodiment, a rolling CRC based on twoor more CRC polynomials and two or more corresponding XOR trees can beprovided (at least on some HPI-compliant devices). For a sequence of twoor more flits, a first CRC code can be generated by the first polynomialfor a first flit. For the second flit, the second CRC polynomial can beused to generate a second CRC code, and so on. The first CRC codegenerated by the first polynomial can be XORed with the second CRC codegenerated by the second polynomial to produce a rolling CRC value. Therolling CRC value can be provided to the receiver (e.g., in the CRCfield of a flit). The rolling CRC value can reflect effectively multipleflits' worth of data improving the ability of the system to detect biterrors of higher burst lengths while no sacrificing additional payloadfor extra CRC bits, among other examples.

In one embodiment, a rolling CRC based on two CRC-16 equations isutilized. Two 16 bit polynomials may be used, the polynomial from HPICRC-16 and a second polynomial. The second polynomial has the smallestnumber of gates to implement a 32 bit rolling CRC algorithm thatrealizes the properties of 1) detection of all 1-7 bit errors; 2) perlane burst protection in x8 link widths (to covers 24 UI in a 8 lanelength); 3) detection of all errors of burst length 16 or less; and 4)only 1:2³² of all other error conditions go undetected. In one example,the second polynomial can comprise 0x10147 (x¹⁶+x⁸+x⁶+x²+x¹+1). Otherexample implementations can utilize the principles illustrated above,such as implementations tailored to flits of a different length, orsystems with links supporting a different (higher or lower) minimum lanewidth with corresponding defined polynomials and CRC field lengths inaccordance with the implementations' particular designs.

HPI can incorporated in any variety of computing devices and systems,including mainframes, server systems, personal computers, mobilecomputers (such as tablets, smartphones, personal digital systems,etc.), smart appliances, gaming or entertainment consoles and set topboxes, among other examples. For instance, FIG. 11 illustrates anexample computer system 1100 in accordance with some implementations. Asshown in FIG. 11, multiprocessor system 1100 is a point-to-pointinterconnect system, and includes a first processor 1170 and a secondprocessor 1180 coupled via a point-to-point interconnect 1150. Each ofprocessors 1170 and 1180 may be some version of a processor. In oneembodiment, 1152 and 1154 are part of a serial, point-to-point coherentinterconnect fabric, such as a high-performance architecture. As aresult, the invention may be implemented within the QPI architecture.

While shown with only two processors 1170, 1180, it is to be understoodthat the scope of the present invention is not so limited. In otherembodiments, one or more additional processors may be present in a givenprocessor.

Processors 1170 and 1180 are shown including integrated memorycontroller units 1172 and 1182, respectively. Processor 1170 alsoincludes as part of its bus controller units point-to-point (P-P)interfaces 1176 and 1178; similarly, second processor 1180 includes P-Pinterfaces 1186 and 1188. Processors 1170, 1180 may exchange informationvia a point-to-point (P-P) interface 1150 using P-P interface circuits1178, 1188. As shown in FIG. 11, IMCs 1172 and 1182 couple theprocessors to respective memories, namely a memory 1132 and a memory1134, which may be portions of main memory locally attached to therespective processors.

Processors 1170, 1180 each exchange information with a chipset 1190) viaindividual P-P interfaces 1152, 1154 using point to point interfacecircuits 1176, 1194, 1186, 1198. Chipset 1190 also exchanges informationwith a high-performance graphics circuit 1138 via an interface circuit1192 along a high-performance graphics interconnect 1139.

A shared cache (not shown) may be included in either processor oroutside of both processors; yet connected with the processors via P-Pinterconnect, such that either or both processors' local cacheinformation may be stored in the shared cache if a processor is placedinto a low power mode.

Chipset 1190 may be coupled to a first bus 1116 via an interface 1196.In one embodiment, first bus 1116 may be a Peripheral ComponentInterconnect (PCI) bus, or a bus such as a PCI Express bus or anotherthird generation I/O interconnect bus, although the scope of the presentinvention is not so limited.

As shown in FIG. 11, various I/O devices 1114 are coupled to first bus1116, along with a bus bridge 1118 which couples first bus 1116 to asecond bus 1120. In one embodiment, second bus 1120 includes a low pincount (LPC) bus. Various devices are coupled to second bus 1120including, for example, a keyboard and/or mouse 1122, communicationdevices 1127 and a storage unit 1128 such as a disk drive or other massstorage device which often includes instructions/code and data 1130, inone embodiment. Further, an audio I/O 1124 is shown coupled to secondbus 1120. Note that other architectures are possible, where the includedcomponents and interconnect architectures vary. For example, instead ofthe point-to-point architecture of FIG. 11, a system may implement amulti-drop bus or other such architecture.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

A design may go through various stages, from creation to simulation tofabrication. Data representing a design may represent the design in anumber of manners. First, as is useful in simulations, the hardware maybe represented using a hardware description language or anotherfunctional description language. Additionally, a circuit level modelwith logic and/or transistor gates may be produced at some stages of thedesign process. Furthermore, most designs, at some stage, reach a levelof data representing the physical placement of various devices in thehardware model. In the case where conventional semiconductor fabricationtechniques are used, the data representing the hardware model may be thedata specifying the presence or absence of various features on differentmask layers for masks used to produce the integrated circuit. In anyrepresentation of the design, the data may be stored in any form of amachine readable medium. A memory or a magnetic or optical storage suchas a disc may be the machine readable medium to store informationtransmitted via optical or electrical wave modulated or otherwisegenerated to transmit such information. When an electrical carrier waveindicating or carrying the code or design is transmitted, to the extentthat copying, buffering, or re-transmission of the electrical signal isperformed, a new copy is made. Thus, a communication provider or anetwork provider may store on a tangible, machine-readable medium, atleast temporarily, an article, such as information encoded into acarrier wave, embodying techniques of embodiments of the presentinvention.

A module as used herein refers to any combination of hardware, software,and/or firmware. As an example, a module includes hardware, such as amicro-controller, associated with a non-transitory medium to store codeadapted to be executed by the micro-controller. Therefore, reference toa module, in one embodiment, refers to the hardware, which isspecifically configured to recognize and/or execute the code to be heldon a non-transitory medium. Furthermore, in another embodiment, use of amodule refers to the non-transitory medium including the code, which isspecifically adapted to be executed by the microcontroller to performpredetermined operations. And as can be inferred, in yet anotherembodiment, the term module (in this example) may refer to thecombination of the microcontroller and the non-transitory medium. Oftenmodule boundaries that are illustrated as separate commonly vary andpotentially overlap. For example, a first and a second module may sharehardware, software, firmware, or a combination thereof, whilepotentially retaining some independent hardware, software, or firmware.In one embodiment, use of the term logic includes hardware, such astransistors, registers, or other hardware, such as programmable logicdevices.

Use of the phrase ‘configured to,’ in one embodiment, refers toarranging, putting together, manufacturing, offering to sell, importingand/or designing an apparatus, hardware, logic, or element to perform adesignated or determined task. In this example, an apparatus or elementthereof that is not operating is still ‘configured to’ perform adesignated task if it is designed, coupled, and/or interconnected toperform said designated task. As a purely illustrative example, a logicgate may provide a 0 or a 1 during operation. But a logic gate‘configured to’ provide an enable signal to a clock does not includeevery potential logic gate that may provide a 1 or 0. Instead, the logicgate is one coupled in some manner that during operation the 1 or 0output is to enable the clock. Note once again that use of the term‘configured to’ does not require operation, but instead focus on thelatent state of an apparatus, hardware, and/or element, where in thelatent state the apparatus, hardware, and/or element is designed toperform a particular task when the apparatus, hardware, and/or elementis operating.

Furthermore, use of the phrases ‘to,’ ‘capable of/to,’ and or ‘operableto,’ in one embodiment, refers to some apparatus, logic, hardware,and/or element designed in such a way to enable use of the apparatus,logic, hardware, and/or element in a specified manner. Note as abovethat use of to, capable to, or operable to, in one embodiment, refers tothe latent state of an apparatus, logic, hardware, and/or element, wherethe apparatus, logic, hardware, and/or element is not operating but isdesigned in such a manner to enable use of an apparatus in a specifiedmanner.

A value, as used herein, includes any known representation of a number,a state, a logical state, or a binary logical state. Often, the use oflogic levels, logic values, or logical values is also referred to as 1'sand 0's, which simply represents binary logic states. For example, a 1refers to a high logic level and 0 refers to a low logic level. In oneembodiment, a storage cell, such as a transistor or flash cell, may becapable of holding a single logical value or multiple logical values.However, other representations of values in computer systems have beenused. For example the decimal number ten may also be represented as abinary value of 1010 and a hexadecimal letter A. Therefore, a valueincludes any representation of information capable of being held in acomputer system.

Moreover, states may be represented by values or portions of values. Asan example, a first value, such as a logical one, may represent adefault or initial state, while a second value, such as a logical zero,may represent a non-default state. In addition, the terms reset and set,in one embodiment, refer to a default and an updated value or state,respectively. For example, a default value potentially includes a highlogical value, i.e. reset, while an updated value potentially includes alow logical value, i.e. set. Note that any combination of values may beutilized to represent any number of states.

The embodiments of methods, hardware, software, firmware or code setforth above may be implemented via instructions or code stored on amachine-accessible, machine readable, computer accessible, or computerreadable medium which are executable by a processing element. Anon-transitory machine-accessible/readable medium includes any mechanismthat provides (i.e., stores and/or transmits) information in a formreadable by a machine, such as a computer or electronic system. Forexample, a non-transitory machine-accessible medium includesrandom-access memory (RAM), such as static RAM (SRAM) or dynamic RAM(DRAM); ROM; magnetic or optical storage medium; flash memory devices;electrical storage devices; optical storage devices; acoustical storagedevices; other form of storage devices for holding information receivedfrom transitory (propagated) signals (e.g., carrier waves, infraredsignals, digital signals); etc, which are to be distinguished from thenon-transitory mediums that may receive information there from.

Instructions used to program logic to perform embodiments of theinvention may be stored within a memory in the system, such as DRAM,cache, flash memory, or other storage. Furthermore, the instructions canbe distributed via a network or by way of other computer readable media.Thus a machine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer), but is not limited to, floppy diskettes, optical disks,Compact Disc, Read-Only Memory (CD-ROMs), and magneto-optical disks,Read-Only Memory (ROMs), Random Access Memory (RAM), ErasableProgrammable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), magnetic or optical cards, flashmemory, or a tangible, machine-readable storage used in the transmissionof information over the Internet via electrical, optical, acoustical orother forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.). Accordingly, the computer-readablemedium includes any type of tangible machine-readable medium suitablefor storing or transmitting electronic instructions or information in aform readable by a machine (e.g., a computer).

The following examples pertain to embodiments in accordance with thisSpecification. One or more embodiments may provide an apparatus, asystem, a machine readable storage, a machine readable medium, and amethod to identify transaction data, generate a flit to include three ormore slots and a floating field to be used as an extension of any one oftwo or more of the slots, and send the flit to transmit the flit.

In at least one example, I/O logic comprises a layered stack comprisingphysical layer logic, link layer logic, and protocol layer logic

In at least one example, the three or more slots consist of threedefined slots.

In at least one example, the flit comprises 192 bits.

In at least one example, the first of the three slots comprises 72 bits,the second of the three slots comprises 70 bits, and third slotcomprises 18 bits

In at least one example, the first slot and second slot each include arespective 50 bit payload field.

In at least one example, the floating field is to extend the payloadfield of either the first slot or the second slot by eleven bits.

In at least one example, the third slot is adapted to be encoded withone or more of acknowledgements and credit returns.

In at least one example, the flit further comprises a 16-bit cyclicredundancy check (CRC) field.

In at least one example, the flit further comprises an 11-bittransaction identifier (TID) field.

In at least one example, each slot is to include a header of a distinctmessage.

In at least one example, each message is associated with a respectivetransaction within a particular virtual network.

In at least one example, the flit further comprises a virtual networkidentifier to identify the particular virtual network.

In at least one example, wherein message headers associated withtransactions in different virtual networks are to be included indistinct flits.

One or more embodiments may provide an apparatus, a system, a machinereadable storage, a machine readable medium, and a method to receive aflit, wherein the flit is to include three or more slots to be includedin the flit and a floating field to be used as an extension of any oneof two or more of the slots, and process each slot to identify one ormore headers relating to one or more transactions.

In at least one example, the one or more headers comprise three or moreheaders.

In at least one example, each of the headers corresponds to a respectivemessage associated with a different, respective transaction.

In at least one example, each of the transactions is included in aparticular virtual network.

In at least one example, it can be identified which of the first andsecond slots the floating field is to extend.

In at least one example, the third slot is adapted to be encoded withone or more of acknowledgements and credit returns.

In at least one example, the flit can be sent from a first device to asecond device over the data link. The first second devices can includemicroprocessors, graphics accelerators, and other devices.

One or more examples can further provide a layered protocol stackincluding at least a link layer and a physical layer, the layeredprotocol stack configured to be coupled to a serial, differential link,wherein the layered protocol stack is further configured to transmit a192-bit flit over the serial, differential link.

In at least one example, the 192-bit flit includes a 16 bit CRC

One or more examples can further provide a layered protocol stackincluding at least a link layer and a physical layer, the layeredprotocol stack configured to be coupled to a serial, differential link,wherein the layered protocol stack is further configured to transmit aflit over the serial, differential link, the flit to include an 11-bittransaction identifier field.

One or more examples can further provide a layered protocol stackincluding at least a link layer and a physical layer, the layeredprotocol stack configured to be coupled to a serial, differential link,wherein the layered protocol stack is further configured to assemble aheader flit including a plurality of slots.

In at least one example, the plurality of payload slots include 3 slots.

In at least one example, the first and second slots of the 3 slots areequal in size and the third slot of the 3 slots is smaller than thefirst slot.

In at least one example, special control flits may consume all 3 slots.

In at least one example, the flit includes a 16 bit CRC.

One or more embodiments may provide an apparatus, a system, a machinereadable storage, a machine readable medium, and a method to identifytransaction data, generate a flit from the transaction data, wherein theflit is to include two or more slots, a payload, and a cyclic redundancycheck (CRC) field to be encoded with a 16-bit CRC value generated basedon the payload, and send the flit to the device over the serial datalink.

In at least one example, the I/O logic comprises a layered stackcomprising physical layer logic, link layer logic, and protocol layerlogic

In at least one example, the two or more slots consist of three definedslots.

In at least one example, the flit comprises 192 bits.

In at least one example, the first of the three slots comprises 72 bits,the second of the three slots comprises 70 bits, and third slotcomprises 18 bits

In at least one example, the third slot is adapted to be encoded withone or more of acknowledgements and credit returns.

In at least one example, the flit payload comprises 176 bits.

In at least one example, the CRC value is generated using an XOR treeand the XOR tree is to embody a generator polynomial. The polynomial cancomprise g(x)=(x16+x15+x13+x12+x10+x9+x8+x7+x6+x4+x3+x1+1). The CRCvalue can be a rolling CRC value.

In at least one example, the data link comprises at least 8 lanes in afirst state and the flit comprises 192 bits.

In at least one example, the first state comprises a partial widthtransmitting state and a full width transmitting state comprises a 20lane link.

One or more embodiments may provide an apparatus, a system, a machinereadable storage, a machine readable medium, and a method to receive aflit, wherein the flit is to include two or more slots, a payload, and acyclic redundancy check (CRC) field encoded with a 16-bit CRC valuegenerated based on the payload, determine a comparison CRC value fromthe payload, and compare the comparison CRC value to the CRC valueincluded in the flit.

In at least one example, one or more errors can be detected on a datalink based on the comparison.

In at least one example, the flit comprises 192 bits, a first of theslots comprises 72 bits, a second of the slots comprises 70 bits, and athird of the slots comprises 18 bits.

In at least one example, the CRC value can be derived using an XOR treethat embodies a generator polynomial. The generator polynomial cancomprises g(x)=(x16+x15+x13+x12+x10+x9+x8+x7+x6+x4+x3+x1+1).

In at least one example, the generator polynomial is the same used togenerate the CRC value included in the flit.

In at least one example, the CRC value comprises a rolling CRC value.

In at least one example, the flit can be sent between a first and seconddevice. The first and second devices can be microprocessors, graphicalaccelerators, or other devices.

One or more examples can further provide a layered protocol stackincluding at least a link layer and a physical layer, the layeredprotocol stack configured to be coupled to a serial, differential link,wherein the layered protocol stack is further configured to calculate arolling CRC for a flit to be transmitted on the link, the rolling CRC tobe based on at least two polynomial equations.

In at least one example, the second polynomial of the two is todetermine if all of 1-7 bit errors are detect, per lane burstprotection, and errors of burst length 16 or less are detected.

One or more examples can further provide a layered protocol stackincluding at least a link layer and a physical layer, the layeredprotocol stack configured to be coupled to a serial, differential link,wherein the layered protocol stack is further configured to assemble aheader flit including a plurality of slots.

In at least one example, the plurality of payload slots include 3 slots.

In at least one example, the first and second slots of the 3 slots areequal in size and the third slot of the 3 slots is smaller than thefirst slot.

In at least one example, the special control flits may consume all 3slots.

In at least one example, the flit includes a 16 bit CRC.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

In the foregoing specification, a detailed description has been givenwith reference to specific exemplary embodiments. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense. Furthermore, the foregoing use of embodiment andother exemplarily language does not necessarily refer to the sameembodiment or the same example, but may refer to different and distinctembodiments, as well as potentially the same embodiment.

What is claimed is:
 1. An apparatus comprising: a host processorcomprising: physical layer logic; and link layer logic, comprisingcircuitry to: generate a flit comprising a plurality of slots, whereinone or more of the slots is encoded with a return credit response, atleast one other one of the plurality of slots is encoded as a null slot,the flit is according to a format to enable a plurality of transactionheaders to be contained in the flit, and the flit comprises a piece of alarger network packet; and a transmitter to send the flit on a link toanother device.
 2. The apparatus of claim 1, wherein a single one of theslots is encoded with the return credit response.
 3. The apparatus ofclaim 1, wherein the other device comprises another host processordevice.
 4. The apparatus of claim 1, wherein the link supports aplurality of lane widths.
 5. The apparatus of claim 4, wherein theplurality of lane widths comprises a lane width of at least 8 lanes. 6.The apparatus of claim 1, wherein the flit comprises at least 192 bits.7. The apparatus of claim 1, wherein the physical layer comprises anelectrical transmission medium.
 8. The apparatus of claim 1, wherein thephysical layer comprises an optical transmission medium.
 9. An apparatuscomprising: a host processor comprising: a receiver to receive a flitfrom another device over a link, wherein the flit comprises a pluralityof slots, one or more of the slots is encoded with a return creditresponse, at least one other one of the plurality of slots is encoded asa null slot, the flit is according to a format to enable a plurality oftransaction headers to be contained in the flit, and the flit comprisesa piece of a larger network packet; a protocol stack comprising physicallayer logic and link layer logic, wherein the link layer logic is toreturn a number of credits to a credit pool based on the return creditresponse.
 10. The apparatus of claim 9, wherein the credit poolcorresponds to a virtual channel of the link.
 11. The apparatus of claim9, wherein a single one of the slots is encoded with the return creditresponse.
 12. The apparatus of claim 9, wherein the other devicecomprises another host processor device.
 13. The apparatus of claim 9,wherein the link supports a plurality of lane widths.
 14. The apparatusof claim 13, wherein the plurality of lane widths comprises a lane widthof at least 8 lanes.
 15. The apparatus of claim 9, wherein the flitcomprises at least 192 bits.
 16. The apparatus of claim 9, wherein thephysical layer comprises an electrical transmission medium.
 17. Theapparatus of claim 9, wherein the physical layer comprises an opticaltransmission medium.
 18. A system comprising: a first host processordevice; a second host processor device to connect to the first hostprocessor device using a link, wherein the second host processor devicecomprises: physical layer logic; and link layer logic, wherein the linklayer logic is to: generate a flit comprising a plurality of slots,wherein a particular one of the plurality of slots is encoded with areturn credit response, another one of the slots in the plurality ofslots is encoded as a null slot, the flit is according to a format toenable a plurality of transaction headers to be contained in the flit,and the flit comprises a piece of a larger network packet; and atransmitter to send the flit on a link to another device.
 19. The systemof claim 18, wherein a single one of the slots is encoded with thereturn credit response.
 20. The system of claim 18, wherein the otherdevice comprises another host processor device.
 21. The system of claim18, wherein the link supports a plurality of lane widths.
 22. The systemof claim 21, wherein the plurality of lane widths comprises a lane widthof at least 8 lanes.
 23. The system of claim 18, wherein the flitcomprises at least 192 bits.
 24. The system of claim 18, wherein thephysical layer comprises an electrical transmission medium.
 25. Thesystem of claim 18, wherein the physical layer comprises an opticaltransmission medium.
 26. A system comprising: means for generating aflit comprising a plurality of slots, wherein one or more of the slotsis encoded with a return credit response, at least one other one of theplurality of slots is encoded as a null slot, the flit is according to aformat to enable a plurality of transaction headers to be contained inthe flit, and the flit comprises a piece of a larger network packet; andmeans for sending the flit on a link to another device.